1. Field of the Invention
The subject matter of this invention is related generally to electromagnetic contactors and more specifically to apparatus for controlling an electromagnetic contactor.
2. Description of the Prior Art
Electromagnetic contactors are well known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is called a motor controller. A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. Disposed on the armature is a set of bridging contacts, the complements of which are fixedly disposed within the contactor case for being engaged thereby as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are usually interconnected with the fixed contacts and become interconnected with each other as the bridging contacts make with the fixed contacts.
When utilizing the system as a motor controller with its overload relay portion, the overload relay function in the prior art is accomplished by using a current transformer which produces an output current which is related to the input current being measured. This then is provided to an electromagnetic or electrothermal device calibrated to cause the main contacts to open under certain overload current situations. In co-pending currently filed U.S. patent application Ser. No. 016,422 and Application Ser. No. 016,420 the overload current relay range is expanded. However, in order for the system to take advantage of the expansion of the range of current sensed, it is necessary when supplying this current to a microprocessor to be able to convert the analog representation thereat to a digital number for effective utilization by the microprocessor. Most microprocessor have an analog-to-digital converter portion therein which requires that the level of the analog representation of the input signal vary within a relatively small range of perhaps 0 to 5 volts. Any signal larger than this will cause saturation of the output locations of the analog-to-digital converter, that is, will cause the output of the analog-to-digital converter to have digital ones in every output location for a typical 8-bit parallel output analog-to-digital converter. Any input signal which is above the 5 volt level will produce an unreliable saturated digital output which cannot be effectively and accurately utilized by the remaining portion of the microprocessor. It would be desirous therefore to provide an electromagnetic controller system in which the wide range of sensed overload current could be effectively utilized by the analog-to-digital converter portion of the microprocessor.